The Structural and Lineage-Based Immunogen Design Scientific Research Support Component (SRSC) has the following roles within the overall B cell immunogen development effort: (a) to acquire a detailed structural picture of the stages of affinity maturation in clonal lineages of HlV-1 protective antibodies derived and analyzed as part of the B Cell Focus and (b) to develop approaches for design of immunogens that can elicit vaccine-induced responses similar to those that have matured in chronically infected subjects with broad neutralizing antibodies. This SRSC will receive BnAb and other protective antibody unmutated ancestor (UA) and intermediate antibodies (IA) from the B Cell Focus and other SRSCs and use UAs and IAs as template for new immunogen design. Specific Aims: Aim 1. Determine crystal structures of critical mAb Fabs from informative clonal antibody lineages from Aims 1 and 4 of the B Cell Focus and the Computational Biology SRSC. Aim 2. Determine crystal structures (when feasible) of selected BnAb or other protective antibody Fabs from B Cell Focus Aims 1 and 4 in complex with gp120 (or a suitable gp120 fragment) or with a gp41 peptide or similar construct. Aim 3. Vary selected positions on gp120 (or gp41) by random mutagenesis and use the library of variants, expressed on the surfaces of individual cells, in integrative screens for antigens that bind with enhanced affinity to UAs or lineage intermediate antibodies. Aim 4. Express Envs or their fragments that result from the screens in Aim 3.